FIELD OF THE INVENTION
The invention relates to a sheet-fed printing press with flat sheet guidance, which has two endless conveyor belts for a rectilinear, intervention-free movement of gripper carriages through the printing units of the press, a device for adjusting the length of the conveyor belts, and a device for synchronizing the speeds of the conveyor belts and the cylinders of the printing press; and to a method for synchronizing sheet travel and a conveyor belt, respectively, with printing-unit cylinders of the press.
A sheet-fed printing press with flat sheet guidance has become known heretofore from German Patent 19 30 317. By flat sheet guidance there is meant that the sheets are passed in a single gripper lock in a horizontal plane from the feeder to the delivery, between the cylinders of a plurality of successive printing units. For in-register printing, sheet transport must be synchronized with the cylinders. Typical transport means with tooth elements, such as chains, however, have a disadvantage in that, during operation, they lengthen, and the tooth elements exhibit more-or-less major pitch errors and cause shocks on entry. Endless conveyor belts without interventions, such as the metal bands or belts proposed in the afore-cited German patent, each of which extends around a drive wheel and a deflection wheel and is kept to its length by suitable prestressing, appear to be more suitable in this respect.
Dispensing with mechanical force synchronization between the transport means and the cylinders creates new problems, however. For example, the drive wheels cannot be manufactured sufficiently precisely and wear out during operation, respectively. Moreover, they expand when warmed. For this reason if no other, synchronization errors occur over time in the gripper carriages secured to the conveyor belts.
To solve this problem, the East German Patent DD 201 865 has proposed that, for example, if the diameter of the drive wheels increases, either the distance from the deflection wheels be increased, so that the belt length remains an integral multiple of the circumference of a drive wheel, or that the rpm of the drive wheels be varied by a suitable variable gear transmission.
For many reasons, however, optimal synchronization cannot be obtained with the device according to the aforementioned East German patent, as is explained quite clearly hereinbelow.